1. Field of the Invention
The present invention relates to the field of packages for semiconductor devices and packaged devices, and more particularly, to the field of high current hermetic packages and hermetically packaged high current devices.
2. Background Information
Semiconductor devices have been packaged in a vast variety of package configurations. These include both hermetic (gas-tight) and non-hermetic (gas-permeable) packages. As the desired operating frequency of a hermetically packaged power device increases, prior art packages begin to present problems. Prior art packages for power devices which are hermetically sealed generally include a metallic case or can. Such metallic cans have device leads which extend through glass seals in the sides of the can. These glass seals bond to both the material of the lead and the material of the can in order to provide the required hermetic seal. The chip in which the device is contained is bonded to the bottom of the can and the contact pads on the top of the device are connected to the through-the-can leads by wire bonds. After the completion of mounting and bonding the chip, a cover or lid is sealed on top of the can to hermetically seal the package. Such packages present a number of disadvantages. First, since the wire bonds are normally made with round wire on the order of about 1 mil (0.025 mm) in diameter for low power integrated circuits and 30-40 mils (0.76-1.02 mm) in diameter for power or high current devices and are about 0.3-0.5 inch (0.76-1.27 cm) long, they have a significant inductance in their own right. Second, the through-the-can leads are round wires about 20-50 mils (1-3 mm) in diameter and about 0.3-0.5 inches (0.76-1.27 cm) long. Thus, these leads also have a significant inductance in their own right. Third, the wires have a significant resistance which adds to the on-resistance of the device. Fourth, in order for the thermal expansion coefficient of the can and the semiconductor device to be sufficiently equal that the device does not become debonded from the can and to prolong the life of the glass seals, the cans are normally made of Kovar.RTM. or Invar.RTM. or other similar low thermal expansion coefficient metals. Such metals are magnetic materials and consequently, have the effect of increasing the inductance of the wire bonds and of the through-the-can leads. Fifth, the glass seals are unreliable over long periods of time and eventually leak. Sixth, the metals used in the can and leads suffer from higher electrical and thermal resistivity than copper. Seventh, such packages have the disadvantage that they are substantially larger in both major surface area and volume than the semiconductor chip and weigh many times what the chip weighs. The net result, is that the final packaged chip is in a heavy, bulky container which has a relatively high inductance. That relatively high inductance is undesirable for high frequency operation of the device, because when coupled with very the high di/dt characteristic of high frequency operation, this inductance leads to very large Ldi/dt voltage disturbances such as overshoots and undershoots, where L is the lead inductance. This problem increases with increasing frequency of operation because inductive effects increase with increasing signal frequency. Such hermetic cans also have the disadvantage that for large area contact pads on the semiconductor device, connection of that pad to the through-the-can lead by a single wire bond can result in excessive lateral currents in the contact pad of the semiconductor device. Consequently, multiple bonds need to be made to the contact pad to limit lateral currents to acceptable values for high current devices.
The above-identified related application discloses a package which overcomes the inductance and size problems of the prior art hermetic packages. However, the provisions of its preferred embodiment for limiting lateral current flow in the contact pad of the device can be improved upon.
There is a need for a compact, hermetic, nonmagnetic package for high frequency power devices which alleviates the problem of high lateral currents in the contact pads of power devices.